The state of the art in oxygen therapy is replete with systems that attempt to conserve oxygen being supplied to a user. The need to conserve oxygen is a result of the understanding that continuous, long term oxygen therapy is expensive because of the large quantities of oxygen that need to be provided to a user. For example, oxygen conservation in aviation applications saves both space and weight.
Less sophisticated oxygen therapy systems provide oxygen at a continuous rate without interruption. The result is that all oxygen supplied to the user during exhalation is wasted. The wasted oxygen can be substantial considering that approximately two thirds of the respiratory cycle is spent in exhalation. Furthermore, these systems do not adjust for rates or depth of inspiration by the user. These factors, combined with the fact that the oxygen therapy is generally being conducted using a mobile oxygen container, demonstrate the need for more prudent oxygen conservation.
As such, more sophisticated systems have been developed which are designed to deliver oxygen only during user inspiration. However, the methods and apparatus for delivering oxygen only during user inspiration met with varying degrees of success.
One method for accomplishing conservation during oxygen therapy is to provide pulsed oxygen delivery to the patient according to some control logic. The control logic can be based upon any number of factors, such as, for example, a prescription, a Federal Aviation Administration (“FAA”) mandated altitude compensated gas flow rate, breathing characteristics of the user, etc. For example, studies support the observation that the first portion of inspiration is the most effective time for oxygen delivery, with the last portion of the user's inspiration not actually providing oxygen to the secondary respiratory system (i.e. the blood stream).
Some systems provide oxygen on demand, where the system is responsive to a patient's breathing. However, these systems have numerous drawbacks which prevent them from serving all the functions of an acceptable oxygen conservation device. These drawbacks include, but are not limited to, a failure to adequately adapt to breathing depth (force) of the person using the system. They also fail to take into account changes that occur due to variations in altitude of the user. They also fail to compensate for changes in battery strength, and fail to adequately conserve battery power.
Other electronic oxygen conserving devices use various methods for sensing the respiration event so that a valve can be triggered to allow oxygen to flow to the user. The sensor to detect breathing events can range from an off-the-shelf low-pressure sensor to a discreetly assembled membrane material based sensor. Many off-the-shelf low-pressure sensors require a second valve to remove the sensor from the pneumatic path to protect it from the high-pressure burst that occurs when oxygen is flowing.
Membrane material based sensors contain a thin flexible membrane that moves in relation to the minute pressure changes caused during breathing. Many of these membrane type sensors are able to handle the pressure of the valve passing the oxygen, but can be expensive to manufacture and then calibrate for use. Electrical signals, corresponding to breathing pressures, are derived from the movements resulting from the minute pressure changes when breathing. The signal from any sensor type is processed so that a valve can be opened at the opportune time.
Electronic Demand Pulsed-Dose oxygen delivery systems deliver oxygen to a user (e.g., a human patient) by detecting the user's (patient's) inspiratory effort and providing gas flow during the initial portion of inspiration. This method reduces the amount of oxygen needed by approximately 50 to 85% (compared to continuous flow) and significantly reduces the cost, the supplies needed, and the limitations on mobility caused by a limited oxygen supply.
As a user initiates a breath, for example, through a cannula tip or mask, the sensor detects the inhalation. In response to detecting inhalation, a solenoid valve opens, and a burst of oxygen is rapidly delivered to the user. The size of the burst or flow can vary. The pulsed-dose system takes the place of a flowmeter during oxygen therapy and is attached to an oxygen source. In most devices, an operator can select the gas flow and the mode of operation (either pulse or continuous flow). A battery-powered fluidic valve is attached to a gaseous or liquid oxygen supply to operate the system.
Other methods used to further reduce oxygen usage when using the pulse-demand system include: reducing the amount of oxygen delivered to the patient during each oxygen pulse and/or to deliver an oxygen pulse only on the second or third breath instead of every breath. In addition, the amount of oxygen in the oxygen pulse can change with the flow setting. Increasing the flow setting can be used to deliver pulses with more oxygen and lowering the flow setting can be used to deliver pulses with less oxygen.
In aviation applications, oxygen distribution can also be delayed until an altitude threshold has been reached.
Potential problems encountered when using the pulse-demand system include either no oxygen flow from the device or decreased oxygen saturation in the patient. If no oxygen flow is detected, then possible causes may include a depletion of the gas supply, an obstruction or disconnection of the connecting tubing, or an inability of the device to detect the patient's effort to breath. If the device cannot detect the patient's inspiratory effort, the sensitivity will need to be increased or the nasal cannula will need to be repositioned in the nares.
A decrease in the patient's oxygen saturation should always be a cause for alarm and may indicate a change in the patient's medical status, tachypnea, or a failure in the device. In any case, a backup system should be available in order to verify whether the problem is with the device or with the patient.
Additionally, pulsed-dose systems are typically costly and have increased complexity. The increased complexity results in a variety of technical problems. For example, pulsed-dose systems may fail to increase oxygen dosage during periods of increased need, such as, for example, exercise, stress, illness, etc. They also have complicated setup procedure which can result in disconnections, improper placement of the device, possible device failures, etc.